The present invention generally relates to electronic video systems or closed circuit television and more particularly relates to a video surveillance apparatus which permits the complete random and independent selection of a plurality of camera signals by a plurality of monitoring stations.
The most elemental video surveillance system consists of a single camera permanently connected through a single coaxial transmission line to a single video monitor. Switching is unnecessary and only a single region may be viewed.
Such an elementary system may be modified to permit surveillance of multiple areas by utilizing a plurality of cameras each connected by coaxial transmission lines to a single viewing station. A simple switching device, such as a rotory switch, may be used to selectively connect any of the cameras to the monitor. The selection is random in that it makes no difference which camera is connected to the monitor. At all times, only one camera is connected to the only monitor allowing the system to be designed for this single condition. Consequently, signal conditions will always be identical. Of course, the signal from the unused cameras is of no consequence.
Problems arise however, when a surveillance system utilizes both multiple cameras and multiple viewing stations or monitors. If each viewing station is permitted to have independent and random access to all camera signals then it is apparent that the number of monitoring stations connected to any given camera can vary anywhere from one to the number of viewing stations in existence.
A problem arises through the existence of multiple cameras and multiple viewing stations. A particular camera may be progressively loaded down as more monitoring stations are connected to it. Thus, not only does the possibility exist that the fan out capacity of the video cameras will be exceeded thereby overloading the camera but also the possibility exists that variations in camera loading will occur even though an overload condition is not reached. There is therefore a need for a video surveillance system which can eliminate variation in the impedance conditions of circuits through which the video signal is coupled.
Ordinarily, in a multi-camera, multi-monitor system the video signal of each camera is fed directly to each viewing station. Each viewing station is also provided with suitable switching equipment for selecting the desired video signal from all fed-in camera signals.
One disadvantage of that system arises because coaxial transmission line is substantially more expensive than multiple conductor cable such as that which is used with low frequency or dc signals. There is therefore a need for a system which minimizes the total length of coaxial transmission line required for any given placement of camera and viewing stations.